The present invention relates to a fail-safe control against a failure of battery voltage for an electronically controlled automatic transmission to be mounted on a vehicle.
In the conventional electronically controlled automatic transmission, the gear ratio of a T/M and a state whether the lock-up clutch is turned on or off are determined depending upon signals each corresponding to the degree of opening of a throttle and a car speed so that an actuator such as solenoid mounted to the T/M is controlled.
However, in the conventional control method, if the voltage of a battery which serves as a power source for the voltage for driving the solenoid is changed due to deterioration of the battery itself or the lack in charge, the driving of the solenoid confronts a problem so that a problem arises that formal gear shift or the state of the lock-up clutch cannot be maintained. In particular, such problems easily occur in cold regions.
Furthermore, in service factories or the like, there is the fear that a 24V battery is erroneously mounted on a 12V-system vehicle, or two 12V batteries are in series connected, each of which causing a high potential to be applied to a solenoid when the engine is started. As a result of which, a failure such as a disconnection can be arisen. Furthermore, problems arise such that remaining a turned-on state by a solenoid becomes impossible when the voltage is lowered, while disconnection failures can be generated due to heat generated by the solenoid when the voltage is raised.